The Collaborative International Dictionary of English v.0.48:

Aliphatic \Al`i*phat"ic\, a. [Gr. ?, ?, oil, fat.] (Org. Chem.)
   Of, pertaining to, or derived from, fat; fatty; -- applied to
   compounds having an open-chain structure. The aliphatic
   compounds thus include not only the fatty acids and other
   derivatives of the paraffin hydrocarbons, but also
   unsaturated compounds, as the ethylene and acetylene series.
   Compare alicyclic and aromatic.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

aliphatic
    adj 1: having carbon atoms linked in open chains